An efficient method of moments for numerical analysis of antennas with variable load impedance is presented. The presented method is a kind of domain decomposition method. A loaded full-matrix equation is decomposed into block matrix equations corresponding to unloaded and loaded parts, respectively. These block matrix equations are solved sequentially using an unloaded full-admittance matrix and currents of the antennas are obtained. The presented method is computationally efficient because matrix inversion of a loaded full-impedance matrix, which is computationally expensive, is unnecessary. The presented method is rigorous because its algorithm includes no approximation. The presented method is applied to numerical analysis of 1-bit transmitarrays and its performance is demonstrated.